1. Field of the Invention
The present invention relates to an electromagnetic flowmeter in which the measurement principle is based on an induced electromotive force due to excitation magnetic fluxes that intersect with fluid to be measured flowing through a pipe, and more particularly to an electromagnetic flowmeter which is improved so as to attain a high magnetic flux density at low power consumption.
2. Description of the Related Art
The following document relates to a measurement principle of an electromagnetic flowmeter having exciting coils of the residual magnetic field type.
JP-A-55-106316 is referred to as a related art.
The following documents relate to a measurement principle of an electromagnetic flowmeter having plural pairs of exciting coils.
JP-A-2001-281028 and JP-A-8-75514 are referred to a related art.
JP-A-2001-281028 discloses an electromagnetic flowmeter which has plural pairs of exciting coils, and which is suited to a large-diameter pipe. However, the electromagnetic flowmeter of JP-A-2001-281028 fails to have a feature of the residual magnetic field type that the operating point is on the permeance line in the second or fourth quadrant of the B-H hysteresis characteristic.
In the electromagnetic flowmeter of JP-A-8-75514 having plural pairs of exciting coils, magnetic circuits are configured by a group of exciting coils applied to plural pole piece cores, and a single return core serving as a feedback magnetic path. However, the configuration does not include plate cores which add together magnetic fluxes of the exciting coils, and evenly distribute the added fluxes to the interior of a pipe to be measured.
Next, an electromagnetic flowmeter corresponding to JP-A-55-106316 will be described with reference to FIGS. 5A and 5B. FIGS. 5A and 5B show in a simplified manner the configuration of an electromagnetic flowmeter having exciting coils of the residual magnetic field type. The electromagnetic flowmeter shown by FIGS. 5A and 5B is configured by magnetic pole cores 1, exciting coils 2, a feedback magnetic path 4, a pipe to be measured 5, and electrodes 6.
The pair of exciting coils 2 which are opposed to each other across the central axis of the flow path of the pipe to be measured 5 are configured so as to form a common magnetic circuit. The exciting coils 2 are connected to a current source which is not shown, and can be simultaneously supplied with exciting currents of the same level, respectively. By contrast, the two electrodes 6 are opposed to each other across the central axis of the flow path of the pipe to be measured 5, and perpendicular to an axis connecting the exciting coils 2 with each other.
The operation of the electromagnetic flowmeter of the residual magnetic field type which is exemplarily shown in FIGS. 5A and 5B will be described. The pair of exciting coils 2 have a function of exciting the magnetic circuit which is not shown. The exciting current in which the direction is intermittently alternately inverted at a constant period is applied to the coils.
In the magnetic pole cores 1, a semi-hard magnetic material having a magnetic property which is between the property of a hard magnetic material (a magnet) and that of a soft magnetic material (iron, silicon iron, or the like) is used.
The winding directions of the exciting coils 2 are set so that, when the exciting current in a certain direction is applied, the exciting coils 2 produce external magnetic fields directed so as to enhance each other. In accordance with the application of the exciting current, therefore, magnetization in both the magnetic pole cores 1 which are positioned at the centers of the respective magnetic fields is advanced, and the magnetic flux densities are saturated.
Even when the exciting current is once extinguished and the external magnetic fields due to the exciting coils 2 disappear, the magnetic pole cores 1 hold a residual magnetic flux density Br. In the electromagnetic flowmeter of the residual magnetic field type, the magnetic circuit in which the residual magnetic flux density provided by the semi-hard magnetic material is used as a magnetomotive force is used in the measurement.
Namely, the magnetic circuit is maintained by the magnetic energy remaining in the two magnetic pole cores 1, and set so that a part of the magnetic path crosses the central axis of the flow path of the pipe to be measured 5. Therefore, the magnetic fluxes intersect with a section of the pipe to be measured 5 so as to be symmetrical about the axis and have a substantially uniform shape.
The magnetic fluxes which have crossed the pipe to be measured 5 are collected in the exciting coil 2 in the opposite pole, and are then fed back through the feedback magnetic path 4 to the exciting coil 2 of the pole on the emerging side, thereby completing the magnetic circuit.
When the exciting current then flows in a direction opposite to that along which the exciting current flows in the previous application, the exciting coils 2 produce magnetic fields in directions along which the magnetic fields enhance each other, and which is opposite to the previous direction. In the magnetic pole cores 1 which are positioned respectively at the centers of the magnetic fields, both the magnetization and the magnetic flux density which are previously acquired disappear after a reduction process, magnetization is then advanced in the opposite direction, and the magnetic flux densities in the opposite direction are saturated. As a result, also the direction of the magnetic circuit passing through the feedback magnetic path 4 and the pipe to be measured 5 is inverted.
The history of the magnetization process of the magnetic pole cores 1 which is repeated at a constant period is known as a magnetic hysteresis curve (B-H curve) or the B-H hysteresis characteristic in which the direction and strength of the applied external magnetic field H are set as the abscissa, and those of the magnetic flux density in the magnetization are set as the ordinate.
In the magnetic pole cores 1, in accordance with the hysteresis characteristic of the semi-hard magnetic material, the magnetization state is alternately inverted through the one cycle process. Also in the magnetic circuit, therefore, the direction of the magnetic path is inverted at a constant period in accordance with the external magnetic field produced at the timing of the exciting current in which the application direction is intermittently alternately inverted at the constant period.
The residual magnetic fluxes of the magnetic pole cores 1 provided by the exciting coils 2 are used as the magnetomotive force which, when the exciting current is not applied, or during the most time period when the inversion of the magnetic path is not conducted, causes the magnetic circuit to maintain the stable magnetic path.
When fluid F in the pipe to be measured 5 crosses the magnetic fluxes of the magnetic circuit in a stabilized state, an induced electromotive force I which has a direction perpendicular to both the magnetic flux density B and the fluid movement F, and a magnitude that is the product of B and F is produced according to Faraday's law of electromagnetic induction.
Because the magnetic pole cores 1 perform the function of holding the residual magnetic flux density, the magnetic flux B at this timing is held to a constant value. Therefore, a voltage which is observed between the electrodes 6 as the induced electromotive force I correctly reflects the degree of the fluid movement F. Namely, the observation of the waveform of the voltage between the electrodes 6 results in real-time observation of the movement distance of the fluid F which is in proportional relationship.
Because of the principle that a physical quantity which is obtained as the product of F·B as described above is used in the measurement, the measurement accuracy and flow velocity resolution of an electromagnetic flowmeter of the residual magnetic field type are directly affected by the residual magnetic flux density B given by an intersecting magnetic circuit.
In the magnetic circuit, the magnetic path is configured as a form which takes a round through the magnetic pole cores 1, the feedback magnetic path 4, air gaps (not shown) in the surfaces of the exciting coils 2, and the like. The magnetic flux density B of a constant value corresponding to the magnetization energy remaining in the semi-hard magnetic material of the magnetic pole cores 1 is determined depending on the permeance Pm of the whole system which is uniquely defined by the qualities, shapes, permeabilities, and the like of materials in the path.
FIG. 6 shows the second quadrant of the B-H hysteresis characteristic of the semi-hard magnetic material used in the magnetic pole cores of the electromagnetic flowmeter of FIGS. 5A and 5B. The permeance Pm which is uniquely defined by the shape and material quality of the magnetic path is constant in the system, and, in FIG. 6, therefore indicated by a permeance line Pm having a constant slope.
A state where a residual magnetization energy required for measuring a flow rate is held means a state where a diamagnetic field component which has a constant ratio with respect to the magnetization energy is balanced and stabilized, and is indicated in the second or fourth quadrant of the B-H hysteresis characteristic. In an electromagnetic flowmeter of the residual magnetic field type, namely, a stabilized state of a system is always attained on the permeance line.
In an electromagnetic flowmeter, as is apparent from the measurement principle, when the magnetic flux density B intersecting with the flow path F can be increased, the accuracy and the flow velocity resolution can be proportionally improved because the measurement result is the product of the constant magnetic flux density B and the flow rate F.
In an electromagnetic flowmeter of the residual magnetic field type, the residual magnetic flux density in a stable state based on the residual magnetic energy is used in measurement, and hence an electric power is consumed only when an external magnetic field required for inverting the magnetic flux direction is applied.
Therefore, the electromagnetic flowmeter of this type consumes less power than an electromagnetic flowmeter of another type, and therefore can be driven by batteries. Consequently, an electromagnetic flowmeter of the residual magnetic field type has an advantage that the installation place is not restricted by the electric power condition and assurance of power supply, and hence is widely used in indoor and outdoor fluid transporting facilities.
In a magnetic circuit of an electromagnetic flowmeter, usually, magnetic fluxes hardly penetrate into an air gap, and hence the function of increasing a diamagnetic field component which is opposite to the direction of a magnetic path with respect to the whole magnetization energy is enhanced. Such a configuration factor causes a function of reducing the permeance Pm obtained in the system. Therefore, it is difficult to largely increase the magnetic flux density Bm.
In an electromagnetic flowmeter of the residual magnetic field type, the permeance Pm is constant. In order to obtain a magnetic flux density which is higher than the current density Bm1 without changing the configuration and structure of a magnetic path, therefore, the material of magnetic pole cores must be changed so as to increase the magnetic coercive force Hc as shown in the characteristic diagram of FIG. 6.
When the magnetic coercive force is increased from Hc to a doubled value or 2·Hc, for example, it is possible to ensure a new operation value Bm2 which is proportionally positioned on the permeance line of FIG. 6 and determined by a residual magnetic flux density Br. Therefore, the operation point of the system is moved from P1 to P2.
In a similar manner as Joule's heat which is generated by the square of a current in an electric circuit, a hysteresis loss is generated in a magnetic circuit in proportion to the square of the magnetic coercive force 2·Hc for excitation. As a result, the required excitation power is four times that required in the case of Hc.
As described above, in an electromagnetic flowmeter of the residual magnetic field type which is characterized in low power consumption, ensuring of the magnetic flux density Bm which is necessary and sufficient for attaining high sensitivity and accuracy with respect to fluid to be measured F is a tradeoff with the power consumption. In the above electromagnetic flowmeter of the residual magnetic field type shown in FIGS. 5A and 5B, namely, it is difficult to solve both the contradictory problems that the power consumption is reduced, and that magnetic fluxes are efficiently increased.